Difference between revisions of "Attribute"

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An '''attribute''' of an object is a piece of data that describes the object, stored within the object as a key-value pair. Given an object, one can ask for the value of a particular key using ''dot notation''.
 
An '''attribute''' of an object is a piece of data that describes the object, stored within the object as a key-value pair. Given an object, one can ask for the value of a particular key using ''dot notation''.
 
<syntaxhighlight lang="python">
 
<syntaxhighlight lang="python">

Revision as of 22:22, 12 July 2014

An attribute of an object is a piece of data that describes the object, stored within the object as a key-value pair. Given an object, one can ask for the value of a particular key using dot notation.

>>> object.key
value

To access an object's list of attributes, just call the dir() function on the object.

Example of handling attributes

>>> import datetime
>>> date_object = datetime.date
>>> dir(date_object)
['__add__', '__class__', '__delattr__', '__doc__', '__eq__', ..., 'today', 'toordinal', 'weekday', 'year']
>>> date.year
2014

Class vs. instance attribute

Class Attribute

Attributes defined within a class but outside of methods are class attributes. Class attributes, which include methods, can be accessed by the class or by instances of the class. Class attributes are 'shared' by all instances and the class itself, unless overridden or overwritten by an instance attribute. When a class attribute is changed, all instances and classes that access it will see the change, regardless of what object made the change. Classes that use assignment statements with dot expressions will set a class attribute.

>>> class Test():
        num = 0
        def do_nothing(self):
            return
>>> Test.do_nothing
<function do_nothing at 0x17d6270>
>>> Test.do_nothing = 1
>>> Test.do_nothing
1
>>> inst = Test()
>>> inst.do_nothing()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'str' object is not callable
>>> inst.do_nothing
1
>>> Test.num
0
>>> inst.num
0

Instance Attribute

Attributes defined inside class methods are instance attributes. Instance attributes are locally defined and can only be accessed by their respective instances. The value of an instance attribute is 'unique' to each instance, and changes to an instance attribute can only be seen by its instance. If a class attribute and instance attribute have the same variable name, then the instance attribute will hide the class attribute. The instance can then only access the instance attribute, although the class can still access the class attribute. Instances that use assignment statements will set instance attributes.

>>> class Test():
        num = 0
        def __init__(self):
            self.num = 1
        def increase(self):
            self.num += 1
>>> Test.num
0
>>> inst1 = Test()
>>> inst2 = Test()
>>> inst1.num
1
>>> inst2.num
1
>>> inst1.increase()
>>> inst1.num
2
>>> inst2.num
1
>>> class Test():
        char = 'a'
>>> Test.char
'a'
>>> inst = Test()
>>> inst.char
'a'
>>> inst.char = 'b'
>>> inst.char
'b'

@property

The property object is, simply put, another way of having a getter and setter method for a defined class. It has three methods a getter, a setter, and a deleter (but don't worry about deleters. They are outside the scope of this course and you will not be tested on them.)

Here's an example:

class ImAClass(object):
 
    def __init__(self):
        self._Property = None
 
 
    @property
    def Property(self):
        """I am a property."""
        return self._Property
 
 
    @Property.setter
    def Property(self, value):
        self._Property = value
 
 
    @Property.deleter
    def Property(self):
        del self._Property

There is little to no difference between implementing a property vs implementing separate getters and setters, and the difference in the amount of code is often negligible.

Here is an example of implementation using getters and setters:

>>> class Get_set_imp(object):
...   val = 0
...   def setter(self, x):
...       self.val = x
...   def getter(self):
...       return self.val
...
 
>>> x = Get_set_imp()
>>> x.setter(3)
>>> x.getter()
3

[1]

And here is the same implementation, this time using properties:

>>> class Prop_imp(object):
...   _val = 0
...   @property
...   def getter(self):
...       return self._val
...   @getter.setter
...   def setter(self, x):
...       self._val = x
...
>>> x = Prop_imp()
>>> x.setter = 10
>>> x.getter
10

Both of these examples essentially do the same thing in roughly the same amount of code. The advantage of having properties however, is that you may switch from the two whenver you want. Simply add or remove or add the property tags from the getters and setters.


AttributeErrors

When one tries to access an attribute of the object that does not exist, Python will raise an AttributeError: 'type_of_obj' object has no attribute 'key'.

'Nonetype' has no attribute

This error occurs when the object to the left of the dot notation is None.

>>> class Foo():
...     def bar(self):
...         print(3)
...
>>> a = Foo()
>>> b = a.bar()
3
>>> b.bar()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'NoneType' object has no attribute 'bar'

Naming conventions

  • _single_leading_underscore indicates that the attribute should be only be used internally by the object.
  • __double_leading_and_trailing__underscores__ denote "magic" attributes, built-in attributes in Python that have a particular meaning beyond just holding a key-value pair.


Sources


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